Golf ball

ABSTRACT

A golf ball  2  includes a center  8 , a mid layer  10 , a cover  6  and dimples  12 . The cover  6  has a Shore D hardness of 30-50. The golf ball  2  has an amount of compressive deformation of 3.0-5.0 mm. The ball  2  meets a mathematical formula (I): 
       0.80≦((L1+L2)/2)≦0.95  (I).
 
     L1 represents a ratio of a lift coefficient CL1 relative to a drag coefficient CD1, the lift coefficient CL1 and the drag coefficient CD1 being measured under conditions of a Reynolds number of 1.290×10 5  and a spin rate of 2820 rpm. L2 represents a ratio of a lift coefficient CL2 relative to a drag coefficient CD2, the lift coefficient CL2 and the drag coefficient CD2 being measured under conditions of a Reynolds number of 1.771×10 5  and a spin rate of 2940 rpm.

This application claims priority on Patent Application No. 2014-133826 filed in JAPAN on Jun. 30, 2014. The entire contents of this Japanese Patent Application are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to golf balls. Specifically, the present invention relates to improvement of aerodynamic characteristics of golf balls.

2. Description of the Related Art

Golf balls have a large number of dimples on the surfaces thereof. The dimples disturb the air flow around the golf ball during flight to cause turbulent flow separation. This phenomenon is referred to as “turbulization”. Due to the turbulization, separation points of the air from the golf ball shift backwards leading to a reduction of drag. The turbulization promotes the displacement between the separation point on the upper side and the separation point on the lower side of the golf ball, which results from the backspin, thereby enhancing the lift force that acts upon the golf ball. Excellent dimples efficiently disturb the air flow. The excellent dimples produce a long flight distance.

A golf player can select the brand of a golf ball to be used by the golf player. In a golf tournament, the brand of a golf ball used by a golf player is often different from that of another golf player. In this respect, golf is a unique ball game.

Golf should be a sport in which golf players compete with each other on their skills. It is not preferable that scores of golf players greatly depend on the brands of golf balls. In this respect, the United States Golf Association (USGA) has established various rules about characteristics of golf balls. For example, rules about weight, diameter, initial speed, flight distance, and symmetry have been established.

Various golf balls have been proposed which can satisfy golf players while conforming to the rules.

JPH5-103846 discloses a golf ball which has dimples the diameters, the depths, and the number of which are made appropriate.

U.S. Pat. No. 5,782,703 (JPH10-43342) discloses a golf ball which has dimples the ratio of the diameter and the depth of each of which is made appropriate.

U.S. Pat. No. 5,782,702 (JPH10-43343) discloses a golf ball in which the ratio of the volumes of dimples relative to the volume of the ball is made appropriate.

JP2000-107338 discloses a golf ball having a diameter and a weight which are made appropriate.

Technological innovation in golf ball and golf club is remarkable. In recent tour tournaments, the average flight distance at tee shots has been increasing. The large flight distance makes a second shot easy. The large flight distance impairs the public interest in tournaments.

The USGA is scheduled to change the rules about flight distance. The head speeds of professional golf players who participate in tournaments are high. The USGA will strengthen regulations on a flight distance upon hitting at a high head speed.

Meanwhile, there are no regulations on a flight distance upon hitting at a low head speed. Amateur golf players desire longer flight distances. A golf ball is desired which achieves a long flight distance when being hit at a low head speed while conforming to the rules of the USGA.

For golf players, in addition to flight distance, feeling is also important. Golf players place importance on hit feeling and trajectory feeling. Golf players prefer soft hit feeling. Furthermore, golf players prefer high trajectories.

An object of the present invention is to provide a golf ball which satisfies a golf player having a low head speed.

SUMMARY OF THE INVENTION

A golf ball according to the present invention includes a core and a cover positioned outside the core. The golf ball has a large number of dimples on a surface thereof. The cover has a Shore D hardness of equal to or greater than 30 but equal to or less than 50. The golf ball has an amount of compressive deformation of equal to or greater than 3.0 mm but equal to or less than 5.0 mm, the amount of compressive deformation being measured under conditions of an initial load of 98 N and a final load of 1274 N. The golf ball meets the following mathematical formula (I):

0.80≦((L1+L2)/2)≧0.95  (I),

where: L1 represents a ratio (CL1/CD1) of a lift coefficient CL1 relative to a drag coefficient CD1, the lift coefficient CL1 and the drag coefficient CD1 being measured under conditions of a Reynolds number of 1.290×10⁵ and a spin rate of 2820 rpm; and L2 represents a ratio (CL2/CD2) of a lift coefficient CL2 relative to a drag coefficient CD2, the lift coefficient CL2 and the drag coefficient CD2 being measured under conditions of a Reynolds number of 1.771×10⁵ and a spin rate of 2940 rpm.

When the golf ball according to the present invention is hit by a golf player having a low head speed, a sufficient flight distance is achieved. The golf ball provides preferable feeling to the golf player having a low head speed.

Preferably, a total volume of the dimples is equal to or greater than 430 mm³ but equal to or less than 580 mm³.

Preferably, the cover has a thickness of equal to or greater than 0.3 mm but equal to or less than 1.8 mm.

Preferably, the ratio L1 is equal to or greater than 0.85 but equal to or less than 0.93. Preferably, the ratio L2 is equal to or greater than 0.76 but equal to or less than 0.92.

Preferably, a ratio of a sum of spherical surface areas of the dimples relative to a surface area of a phantom sphere of the golf ball is equal to or greater than 0.780 but equal to or less than 0.950.

Preferably, a total number of the dimples is equal to or greater than 250 but equal to or less than 450.

Preferably, each dimple has a diameter Dm of equal to or greater than 2.0 mm but equal to or less than 6.0 mm.

Preferably, each dimple has a depth of equal to or greater than 0.10 mm but equal to or less than 0.60 mm.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a golf ball according to one embodiment of the present invention;

FIG. 2 is an enlarged front view of the golf ball in FIG. 1;

FIG. 3 is a plan view of the golf ball in FIG. 2; and

FIG. 4 is a partially enlarged cross-sectional view of the golf ball in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following will describe in detail the present invention, based on preferred embodiments with reference to the accompanying drawings.

FIG. 1 is a partially cutaway cross-sectional view of a golf ball 2 according to one embodiment of the present invention. The golf ball 2 includes a spherical core 4 and a cover 6 positioned outside the core 4. The core 4 includes a spherical center 8 and a mid layer 10 positioned outside the center 8. The golf ball 2 includes a paint layer and a mark layer on the external side of the cover 6 although these layers are not shown in the drawing. Furthermore, the golf ball 2 has a large number of dimples 12 on the surface thereof. Of the surface of the golf ball 2, the part other than the dimples 12 is a land 14. The golf ball 2 may include another layer between the center 8 and the mid layer 10. The golf ball 2 may include another layer between the mid layer 10 and the cover 6.

The golf ball 2 preferably has a diameter of equal to or greater than 40 mm but equal to or less than 45 mm. From the standpoint of conformity to the rules established by the United States Golf Association (USGA), the diameter is particularly preferably equal to or greater than 42.67 mm. In light of suppression of air resistance, the diameter is more preferably equal to or less than 44 mm and particularly preferably equal to or less than 42.80 mm. The golf ball 2 preferably has a weight of equal to or greater than 40 g but equal to or less than 50 g. In light of attainment of great inertia, the weight is more preferably equal to or greater than 44 g and particularly preferably equal to or greater than 45.00 g. From the standpoint of conformity to the rules established by the USGA, the weight is particularly preferably equal to or less than 45.93 g.

The center 8 is obtained by crosslinking a rubber composition. Examples of preferable base rubbers for use in the rubber composition include polybutadienes, polyisoprenes, styrene-butadiene copolymers, ethylene-propylene-diene copolymers, and natural rubbers. In light of resilience performance, polybutadienes are preferred. When a polybutadiene and another rubber are used in combination, it is preferred if the polybutadiene is a principal component. Specifically, the proportion of the polybutadiene to the entire base rubber is preferably equal to or greater than 50% by weight and particularly preferably equal to or greater than 80% by weight. A polybutadiene in which the proportion of cis-1,4 bonds is equal to or greater than 80% is particularly preferred.

The rubber composition of the center 8 preferably includes a co-crosslinking agent. Preferable co-crosslinking agents in light of resilience performance are monovalent or bivalent metal salts of an α,β-unsaturated carboxylic acid having 2 to 8 carbon atoms. Examples of preferable co-crosslinking agents include zinc acrylate, magnesium acrylate, zinc methacrylate, and magnesium methacrylate. In light of resilience performance, zinc acrylate and zinc methacrylate are particularly preferred.

The rubber composition may include a metal oxide and an α,β-unsaturated carboxylic acid having 2 to 8 carbon atoms. They both react with each other in the rubber composition to obtain a salt. The salt serves as a co-crosslinking agent. Examples of preferable α,β-unsaturated carboxylic acids include acrylic acid and methacrylic acid. Examples of preferable metal oxides include zinc oxide and magnesium oxide.

In light of resilience performance of the golf ball 2, the amount of the co-crosslinking agent per 100 parts by weight of the base rubber is preferably equal to or greater than 10 parts by weight and particularly preferably equal to or greater than 15 parts by weight. In light of soft feel at impact, the amount is preferably equal to or less than 50 parts by weight and particularly preferably equal to or less than 45 parts by weight.

Preferably, the rubber composition of the center 8 includes an organic peroxide together with the co-crosslinking agent. The organic peroxide serves as a crosslinking initiator. The organic peroxide contributes to the resilience performance of the golf ball 2. Examples of suitable organic peroxides include dicumyl peroxide, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, and di-t-butyl peroxide. An organic peroxide with particularly high versatility is dicumyl peroxide.

In light of resilience performance of the golf ball 2, the amount of the organic peroxide per 100 parts by weight of the base rubber is preferably equal to or greater than 0.1 parts by weight, more preferably equal to or greater than 0.3 parts by weight, and particularly preferably equal to or greater than 0.5 parts by weight. In light of soft feel at impact, the amount is preferably equal to or less than 3.0 parts by weight, more preferably equal to or less than 2.8 parts by weight, and particularly preferably equal to or less than 2.5 parts by weight.

Preferably, the rubber composition of the center 8 includes an organic sulfur compound. Examples of preferable organic sulfur compounds include monosubstitutions such as diphenyl disulfide, bis(4-chlorophenyl)disulfide, bis(3-chlorophenyl)disulfide, bis(4-bromophenyl)disulfide, bis(3-bromophenyl)disulfide, bis(4-fluorophenyl)disulfide, bis(4-iodophenyl)disulfide, and bis(4-cyanophenyl)disulfide; disubstitutions such as bis(2,5-dichlorophenyl)disulfide, bis(3,5-dichlorophenyl)disulfide, bis(2,6-dichlorophenyl)disulfide, bis(2,5-dibromophenyl)disulfide, bis(3,5-dibromophenyl)disulfide, bis(2-chloro-5-bromophenyl)disulfide, and bis(2-cyano-5-bromophenyl)disulfide; trisubstitutions such as bis(2,4,6-trichlorophenyl)disulfide and bis(2-cyano-4-chloro-6-bromophenyl)disulfide; tetrasubstitutions such as bis(2,3,5,6-tetrachlorophenyl)disulfide; and pentasubstitutions such as bis(2,3,4,5,6-pentachlorophenyl)disulfide and bis(2,3,4,5,6-pentabromophenyl)disulfide. The organic sulfur compound contributes to resilience performance. Particularly preferable organic sulfur compounds are diphenyl disulfide and bis(pentabromophenyl)disulfide.

In light of resilience performance of the golf ball 2, the amount of the organic sulfur compound per 100 parts by weight of the base rubber is preferably equal to or greater than 0.1 parts by weight and particularly preferably equal to or greater than 0.2 parts by weight. In light of soft feel at impact, the amount is preferably equal to or less than 1.5 parts by weight, more preferably equal to or less than 1.0 parts by weight, and particularly preferably equal to or less than 0.8 parts by weight.

For the purpose of adjusting specific gravity and the like, a filler may be included in the center 8. Examples of suitable fillers include zinc oxide, barium sulfate, calcium carbonate, and magnesium carbonate. Powder of a metal with a high specific gravity may be included as a filler. Specific examples of metals with a high specific gravity include tungsten and molybdenum. The amount of the filler is determined as appropriate so that the intended specific gravity of the center 8 is accomplished. A particularly preferable filler is zinc oxide. Zinc oxide serves not only as a specific gravity adjuster but also as a crosslinking activator. According to need, various additives such as sulfur, an anti-aging agent, a coloring agent, a plasticizer, a dispersant, and the like are included in the rubber composition of the center 8 in an adequate amount. Crosslinked rubber powder or synthetic resin powder may also be included in the center 8.

The center 8 preferably has a surface hardness H1 of equal to or greater than 35 but equal to or less than 60. The center 8 having a surface hardness H1 of equal to or greater than 35 can achieve excellent resilience performance. In this respect, the surface hardness H1 is more preferably equal to or greater than 40 and particularly preferably equal to or greater than 45. The center 8 having a surface hardness H1 of equal to or less than 60 can achieve excellent feel at impact. In this respect, the surface hardness H1 is more preferably equal to or less than 55 and particularly preferably equal to or less than 50. The surface hardness H1 is measured by pressing a Shore D type hardness scale against the surface of the center 8 from which the mid layer 10 and the cover 6 have been removed. For the measurement, an automated rubber hardness measurement machine (trade name “P1”, manufactured by Kobunshi Keiki Co., Ltd.), to which this hardness scale is mounted, is used.

In light of feel at impact, the center 8 has an amount of compressive deformation DF1 of preferably equal to or greater than 3.5 mm, more preferably equal to or greater than 4.0 mm, and particularly preferably equal to or greater than 4.5 mm. In light of resilience performance, the amount of compressive deformation DF1 is preferably equal to or less than 6.5 mm, more preferably equal to or less than 6.0 mm, and particularly preferably equal to or less than 5.5 mm,

For measurement of the amount of compressive deformation, a YAMADA type compression tester is used. In the tester, a sphere (the center 8, the core 4, the golf ball 2, etc.) which is an object to be measured is placed on a hard plate made of metal. Next, a cylinder made of metal gradually descends toward the sphere. The sphere, squeezed between the bottom face of the cylinder and the hard plate, becomes deformed. A migration distance of the cylinder, starting from the state in which an initial load of 98 N is applied to the sphere up to the state in which a final load of 1274 N is applied thereto, is measured. A moving speed of the cylinder until the initial load is applied is 0.83 mm/s, A moving speed of the cylinder after the initial speed is applied until the final load is applied is 1.67 mm/s. The atmospheric temperature at the measurement is 23° C. Prior to the measurement, the sphere is kept in a thermostat bath at 23° C. for 24 hours or longer.

The center 8 preferably has a diameter of equal to or greater than 35.0 mm but equal to or less than 40.0 mm. The center 8 preferably has a weight of equal to or greater than 30 g but equal to or less than 41 g. The temperature for crosslinking the center 8 is equal to or higher than 140° C. but equal to or lower than 180° C. The time period for crosslinking the center 8 is equal to or longer than 10 minutes but equal to or shorter than 60 minutes. The center 8 may include two or more layers. The center 8 may have a rib on the surface thereof. The center 8 may be hollow.

The mid layer 10 is formed from a thermoplastic resin composition. Examples of the base polymer of the resin composition include ionomer resins, thermoplastic polyester elastomers, thermoplastic polyamide elastomers, thermoplastic polyurethane elastomers, thermoplastic polyolefin elastomers, and thermoplastic polystyrene elastomers. Ionomer resins are particularly preferred. Ionomer resins are highly elastic. As described later, the cover 6 of the golf ball 2 is thin. When the golf ball 2 is hit, the mid layer 10 significantly deforms due to the thinness of the cover 6. Therefore, the mid layer 10 greatly influences resilience performance. The golf ball 2 which includes the mid layer 10 including an ionomer resin has excellent resilience performance.

An ionomer resin and another resin may be used in combination. In this case, in light of resilience performance, the ionomer resin is included as the principal component of the base polymer. The proportion of the ionomer resin to the entire base polymer is preferably equal to or greater than 50% by weight, more preferably equal to or greater than 70% by weight, and particularly preferably equal to or greater than 85% by weight.

Examples of preferable ionomer resins include binary copolymers formed with an α-olefin and an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms. A preferable binary copolymer includes 80% by weight or more but 90% by weight or less of an α-olefin, and 10% by weight or more but 20% by weight or less of an α,β-unsaturated carboxylic acid. The binary copolymer has excellent resilience performance. Examples of other preferable ionomer resins include ternary copolymers formed with: an α-olefin; an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms; and an α,β-unsaturated carboxylate ester having 2 to 22 carbon atoms. A preferable ternary copolymer includes 70% by weight or more but 85% by weight or less of an α-olefin, 5% by weight or more but 30% by weight or less of an α,β-unsaturated carboxylic acid, and 1% by weight or more but 25% by weight or less of an α,β-unsaturated carboxylate ester. The ternary copolymer has excellent resilience performance. For the binary copolymer and the ternary copolymer, preferable α-olefins are ethylene and propylene, while preferable α,β-unsaturated carboxylic acids are acrylic acid and methacrylic acid. A particularly preferable ionomer resin is a copolymer formed with ethylene and acrylic acid. Another particularly preferable ionomer resin is a copolymer formed with ethylene and methacrylic acid.

In the binary copolymer and the ternary copolymer, some of the carboxyl groups are neutralized with metal ions. Examples of metal ions for use in neutralization include sodium ion, potassium ion, lithium ion, zinc ion, calcium ion, magnesium ion, aluminum ion, and neodymium ion. The neutralization may be carried out with two or more types of metal ions. Particularly suitable metal ions in light of resilience performance and durability of the golf ball 2 are sodium ion, zinc ion, lithium ion, and magnesium ion.

Specific examples of ionomer resins include trade names “Himilan 1555”, “Himilan 1557”, “Himilan 1605”, “Himilan 1706”, “Himilan 1707”, “Himilan 1856”, “Himilan 1855”, “Himilan AM7311”, “Himilan AM7315”, “Himilan AM7317”, “Himilan AM7329”, and “Himilan AM7337”, manufactured by Du Pont-MITSUI POLYCHEMICALS Co., Ltd.; trade names “Surlyn 6120”, “Surlyn 6910”, “Surlyn 7930”, “Surlyn 7940”, “Surlyn 8140”, “Surlyn 8150”, “Surlyn 8940”, “Surlyn 8945”, “Surlyn 9120”, “Surlyn 9150”, “Surlyn 9910”, “Surlyn 9945”, “Surlyn AD8546”, “HPF1000”, and “HPF2000”, manufactured by E.I. du Pont de Nemours and Company; and trade names “IOTEK 7010”, “IOTEK 7030”, “IOTEK 7510”, “IOTEK 7520”, “IOTEK 8000”, and “IOTEK 8030”, manufactured by ExxonMobil Chemical Corporation. Two or more ionomer resins may be used in combination.

For the purpose of adjusting specific gravity and the like, a filler may be included in the resin composition of the mid layer 10. Examples of suitable fillers include zinc oxide, barium sulfate, calcium carbonate, and magnesium carbonate. Powder of a metal with a high specific gravity may be included as a filler. Specific examples of metals with a high specific gravity include tungsten and molybdenum. The amount of the filler is determined as appropriate so that the intended specific gravity of the mid layer 10 is accomplished. A coloring agent, crosslinked rubber powder, or synthetic resin powder may also be included in the mid layer 10.

The mid layer 10 has a thickness of preferably equal to or greater than 0.5 mm and more preferably equal to or greater than 1.0 mm. The thickness is preferably equal to or less than 2.0 mm and more preferably equal to or less than 1.8 mm.

The mid layer 10 has a hardness H2 of preferably equal to or greater than 55, more preferably equal to or greater than 60, and particularly preferably equal to or greater than 63. The hardness H2 is preferably equal to or less than 75, more preferably equal to or less than 72, and particularly preferably equal to or less than 70.

In the present invention, the hardness H2 of the mid layer 10 and a hardness H3 of the cover 6 are measured according to the standards of “ASTM-D 2240-68”. For the measurement, an automated rubber hardness measurement machine (trade name “P1”, manufactured by Kobunshi Keiki Co., Ltd.), to which a Shore D type hardness scale is mounted, is used. For the measurement, a sheet that is formed by hot press, is formed from the same material as that of the mid layer 10 (or the cover 6), and has a thickness of about 2 mm is used. Prior to the measurement, a sheet is kept at 23° C. for two weeks. At the measurement, three sheets are stacked.

The core 4 (i.e., the center 8 and the mid layer 10) preferably has a diameter of equal to or greater than 41.0 mm but equal to or less than 42.0 mm. The core 4 preferably has a weight of equal to or greater than 42.0 g but equal to or less than 44.0 g. The core 4 preferably has an amount of compressive deformation DF2 of equal to or greater than 3.0 mm but equal to or less than 5.0 mm.

The cover 6 is formed from a resin composition. Examples of the base resin of the resin composition include polyurethanes, polyamide elastomers, styrene block-containing thermoplastic elastomers, polyester elastomers, polyolefin elastomers, and ionomer resins.

A preferable base polymer is a polyurethane. The resin composition may include a thermoplastic polyurethane, or may include a thermosetting polyurethane. In light of productivity, the thermoplastic polyurethane is preferable. The thermoplastic polyurethane includes a polyurethane component as a hard segment, and a polyester component or a polyether component as a soft segment. The thermoplastic polyurethane is flexible. The cover 6 in which the polyurethane is used has excellent scuff resistance. When a thermoplastic polyurethane and another resin are used in combination for the cover 6, the proportion of the thermoplastic polyurethane to the entire base resin is preferably equal to or greater than 50% by weight, more preferably equal to or greater than 60% by weight, and particularly preferably equal to or greater than 70% by weight.

The thermoplastic polyurethane has a urethane bond within the molecule. The urethane bond can be formed by reacting a polyol with a polyisocyanate. The polyol, as a material for the urethane bond, has a plurality of hydroxyl groups. Low-molecular-weight polyols and high-molecular-weight polyols can be used.

Examples of low-molecular-weight polyols include diols, triols, tetraols, and hexaols. Specific examples of diols include ethylene glycol, diethylene glycol, triethylene glycol, 1,2-propanediol, 1,3-propanediol, 2-methyl-1,3-propanediol, dipropylene glycol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 2,3-butanediol, 2,3-dimethyl-2,3-butanediol, neopentyl glycol, pentanediol, hexanediol, heptanediol, octanediol, and 1,6-cyclohexanedimethylol. Aniline-based diols or bisphenol A-based diols may be used. Specific examples of triols include glycerin, trimethylol propane, and hexanetriol. Specific examples of tetraols include pentaerythritol and sorbitol.

Examples of high-molecular-weight polyols include polyether polyols such as polyoxyethylene glycol (PEG), polyoxypropylene glycol (PPG), and polytetramethylene ether glycol (PTMG); condensed polyester polyols such as polyethylene adipate (PEA), polybutylene adipate (PBA), and polyhexamethylene adipate (PHMA); lactone polyester polyols such as poly-ε-caprolactone (PCL); polycarbonate polyols such as polyhexamethylene carbonate; and acrylic polyols. Two or more polyols may be used in combination. In light of feel at impact of the golf ball 2, the high-molecular-weight polyol has a number average molecular weight of preferably equal to or greater than 400 and more preferably equal to or greater than 1000. The number average molecular weight is preferably equal to or less than 10000.

Examples of polyisocyanates, as a material for the urethane bond, include aromatic diisocyanates, alicyclic diisocyanates, and aliphatic diisocyanates. Two or more types of diisocyanates may be used in combination,

Examples of aromatic diisocyanates include 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, 4,4′-diphenylmethane diisocyanate (MDI), 1,5-naphthylene diisocyanate (NDI), 3,3′-bitolylene-4,4′-diisocyanate (TODI), xylylene diisocyanate (XDI), tetramethylxylylene diisocyanate (TMXDI), and paraphenylene diisocyanate (PPDI). One example of aliphatic diisocyanates is hexamethylene diisocyanate (HDI). Examples of alicyclic diisocyanates include 4,4′-dicyclohexylmethane diisocyanate (H₁₂MDI), 1,3-bis(isocyanatemethyl)cyclohexane (H₆XDI) isophorone diisocyanate (IPDI), and trans-1,4-cyclohexane diisocyanate (CHDI). 4,4′-dicyclohexylmethane diisocyanate is preferable.

Specific examples of the thermoplastic polyurethane include trade names “Elastollan XNY80A”, “Elastollan XNY82A”, “Elastollan XNY85A”, “Elastollan XNY90A”, “Elastollan XNY97A”, “Elastollan XNY585”, and “Elastollan XKP016N”, manufactured by BASF Japan Ltd.; and trade names “RESAMINE P4585LS” and “RESAMINE PS62490”, manufactured by Dainichiseika Color & Chemicals Mfg. Co., Ltd.

According to need, a coloring agent such as titanium dioxide, a filler such as barium sulfate, a dispersant, an antioxidant, an ultraviolet absorber, a light stabilizer, a fluorescent material, a fluorescent brightener, and the like are included in the resin composition of the cover 6 in an adequate amount.

The cover 6 preferably has a hardness H3 of equal to or greater than 30 but equal to or less than 50. The golf ball 2 having a hardness H3 of equal to or greater than 30 has excellent resilience performance. When the golf ball 2 is hit by an amateur golf player, a long flight distance is achieved. In this respect, the hardness H3 is more preferably equal to or greater than 31 and particularly preferably equal to or greater than 32. The golf ball 2 having a hardness H3 of equal to or less than 50 has excellent spin performance. The spin achieves a high trajectory of the golf ball 2. When the golf ball 2 is hit by an amateur golf player, excellent trajectory feeling is obtained. In this respect, the hardness H3 is more preferably equal to or less than 45 and particularly preferably equal to or less than 40.

The cover 6 preferably has a thickness of equal to or greater than 0.3 mm but equal to or less than 1.8 mm. The golf ball 2 in which the thickness of the cover 6 is equal to or greater than 0.3 mm has excellent spin performance. The spin achieves a high trajectory of the golf ball 2. When the golf ball 2 is hit by an amateur golf player, excellent trajectory feeling is obtained. In this respect, the thickness is more preferably equal to or greater than 0.4 mm and particularly preferably equal to or greater than 0.5 mm. The golf ball 2 in which the thickness is equal to or less than 1.8 mm has excellent resilience performance. When the golf ball 2 is hit by an amateur golf player, a long flight distance is achieved. Furthermore, the golf ball 2 in which the thickness of the cover 6 is equal to or less than 1.8 mm is excellent in hit feeling. In these respects, the thickness is more preferably equal to or less than 1.0 mm and particularly preferably equal to or less than 0.8 mm.

The golf ball 2 may include a reinforcing layer between the mid layer 10 and the cover 6. The reinforcing layer firmly adheres to the mid layer 10 and also to the cover 6. The reinforcing layer suppresses separation of the mid layer 10 from the cover 6. The reinforcing layer is formed from a resin composition. Examples of a preferable base polymer of the reinforcing layer include two-component curing type epoxy resins and two-component curing type urethane resins.

The golf ball 2 has an amount of compressive deformation DF3 of equal to or greater than 3.0 mm but equal to or less than 5.0 mm. The golf ball 2 having an amount of compressive deformation DF3 of equal to or greater than 3.0 mm does not have excessive resilience performance. The golf ball 2 can conform to the rules about flight distance established by the USGA. Furthermore, with the golf ball 2 having an amount of compressive deformation DF3 of equal to or greater than 3.0 mm, soft hit feeling is obtained. In these respects, the amount of compressive deformation DF3 is more preferably equal to or greater than 3.5 mm and particularly preferably equal to or greater than 3.8 mm. The golf ball 2 having an amount of compressive deformation DF3 of equal to or less than 5.0 mm has excellent resilience performance. In this respect, the amount of compressive deformation DF3 is more preferably equal to or less than 4.7 mm and particularly preferably equal to or less than 4.5 mm.

As shown in FIGS. 2 and 3, the contour of each dimple 12 is circular. The golf ball 2 has dimples A each having a diameter of 4.6 mm; dimples B each having a diameter of 4.4 mm; dimples C each having a diameter of 4.2 mm; dimples D each having a diameter of 4.0 mm; dimples E each having a diameter of 3.9 mm; and dimples F each having a diameter of 2.6 mm. The number of types of the dimples 12 is six. The golf ball 2 may have non-circular dimples instead of the circular dimples 12 or together with circular dimples 12.

The number of the dimples A is 42; the number of the dimples B is 72; the number of the dimples C is 66; the number of the dimples D is 126; the number of the dimples E is 12; and the number of the dimples F is 12. The total number of the dimples 12 is 330.

FIG. 4 shows a cross section along a plane passing through the center of the dimple 12 and the center of the golf ball 2. In FIG. 4, the top-to-bottom direction is the depth direction of the dimple 12. In FIG. 4, a chain double-dashed line indicates a phantom sphere 16. The surface of the phantom sphere 16 is the surface of the golf ball 2 when it is postulated that no dimple 12 exists. The dimple 12 is recessed from the surface of the phantom sphere 16. The land 14 coincides with the surface of the phantom sphere 16. In the present embodiment, the cross-sectional shape of each dimple 12 is substantially a circular arc.

In FIG. 4, a double ended arrow Dm indicates the diameter of the dimple 12. The diameter Dm is the distance between two tangent points Ed appearing on a tangent line Tg that is drawn tangent to the far opposite ends of the dimple 12. Each tangent point Ed is also the edge of the dimple 12. The edge Ed defines the contour of the dimple 12. In FIG. 4, a double ended arrow Dp indicates the depth of the dimple 12. The depth Dp is the distance between the deepest part of the dimple 12 and the phantom sphere 16.

The diameter Dm of each dimple 12 is preferably equal to or greater than 2.0 mm but equal to or less than 6.0 mm. The dimple 12 having a diameter Dm of equal to or greater than 2.0 mm contributes to turbulization. In this respect, the diameter Dm is more preferably equal to or greater than 2.5 mm and particularly preferably equal to or greater than 2.8 mm. The dimple 12 having a diameter Dm of equal to or less than 6.0 mm does not impair a fundamental feature of the golf ball 2 that the golf ball 2 is substantially a sphere. In this respect, the diameter Dm is more preferably equal to or less than 5.5 mm and particularly preferably equal to or less than 5.0 mm.

In light of suppression of rising of the golf ball 2 during flight, the depth Dp of each dimple 12 is preferably equal to or greater than 0.10 mm, more preferably equal to or greater than 0.13 mm, and particularly preferably equal to or greater than 0.15 mm. In light of suppression of dropping of the golf ball 2 during flight, the depth Dp is preferably equal to or less than 0.60 mm, more preferably equal to or less than 0.55 mm, and particularly preferably equal to or less than 0.50 mm.

The spherical surface area s of each dimple 12 is the area of a zone surrounded by the contour line of the dimple 12, of the surface of the phantom sphere 16 of the golf ball 2. In the golf ball 2 shown in FIGS. 2 and 3, the spherical surface area s of each dimple A is 16.61 mm²; the spherical surface area s of each dimple B is 15.20 mm²; the spherical surface area s of each dimple C is 13.85 mm²; the spherical surface area s of each dimple D is 12.56 mm²; the spherical surface area s of each dimple E is 11.94 mm²; and the spherical surface area s of each dimple F is 5.31 mm².

The ratio of the sum of the spherical surface areas s of all the dimples 12 to the surface area of the phantom sphere 16 is referred to as an occupation ratio. In light of turbulization, the occupation ratio is preferably equal to or greater than 0.780, more preferably equal to or greater than 0.800, and particularly preferably equal to or greater than 0.840. The occupation ratio is preferably equal to or less than 0.950. In the golf ball 2 shown in FIGS. 2 and 3, the sum of the spherical surface areas s is 4495.3 mm². The surface area of the phantom sphere 16 of the golf ball 2 is 5728.0 mm², and thus the occupation ratio is 0.785.

From the standpoint that a sufficient occupation ratio is achieved, the total number of the dimples 12 is preferably equal to or greater than 250, more preferably equal to or greater than 280, and particularly preferably equal to or greater than 300. From the standpoint that each dimple 12 can contribute to turbulization, the total number of the dimples 12 is preferably equal to or less than 450, more preferably equal to or less than 400, and particularly preferably equal to or less than 380.

In the present invention, the “volume of the dimple” means the volume of a portion surrounded by the phantom sphere 16 and the surface of the dimple 12. The total volume of the dimples 12 of the golf ball 2 is preferably equal to or less than 580 mm³. With the golf ball 2 in which the total volume is equal to or less than 580 mm³, a high trajectory is achieved. When the golf ball 2 is hit by an amateur golf player, excellent trajectory feeling is obtained. In this respect, the total volume is more preferably equal to or less than 560 mm³ and particularly preferably equal to or less than 550 mm³. In light of suppression of rising of the golf ball 2 during flight, the total volume is preferably equal to or greater than 430 mm³.

The golf ball 2 meets the following mathematical formula (I):

0.80≦((L1+L2)/2)≦0.95  (I),

where: L1 represents the ratio (CL1/CD1) of a lift coefficient CL1 relative to a drag coefficient CD1, the lift coefficient CL1 and the drag coefficient CD1 being measured under conditions of a Reynolds number of 1.290×10⁵ and a spin rate of 2820 rpm; and L2 represents the ratio (CL2/CD2) of a lift coefficient CL2 relative to a drag coefficient CD2, the lift coefficient CL2 and the drag coefficient CD2 being measured under conditions of a Reynolds number of 1.771×10⁵ and a spin rate of 2940 rpm. The lift coefficients and the drag coefficients are measured according to the Indoor Test Range (ITR) determined by the USGA.

A Reynolds number is a dimensionless number used in the field of fluid mechanics. A Reynolds number (Re) can be calculated by the following mathematical formula:

Re=ρvL/μ,

where, ρ represents the density of a fluid, v represents a speed of an object, L represents a characteristic length, and μ represents a viscosity coefficient of the fluid.

As described above, the Reynolds number at the measurements of the lift coefficient CL1 and the drag coefficient CD1 is 1.290×10⁵. Regarding the golf ball 2 which flies in the air, this Reynolds number corresponds to a ball speed when the golf ball 2 is launched with a driver at a head speed of 35 m/s. The spin rate at the measurements of the lift coefficient CL1 and the drag coefficient CD1 is 2820 rpm. This spin rate is an average value of a golf player having a head speed of 35 m/s. For measurement of the ratio L1, a golf player having a head speed of 35 m/s is assumed.

As described above, the Reynolds number at the measurements of the lift coefficient CL2 and the drag coefficient CD2 is 1.771×10⁵. Regarding the golf ball 2 which flies in the air, this Reynolds number corresponds to a ball speed when the golf ball 2 is launched with a driver at a head speed of 45 m/s. The spin rate at the measurements of the lift coefficient CL2 and the drag coefficient CD2 is 2940 rpm. This spin rate is an average value of a golf player having a head speed of 45 m/s. For measurement of the ratio L2, a golf player having a head speed of 45 m/s is assumed.

Many amateur golf players have a head speed of equal to or greater than 35 m/s but equal to or less than 45 m/s. In the above mathematical formula (I), the ratio L1 corresponding to a head speed of 35 m/s and the ratio L2 corresponding to a head speed of 45 m/s are averaged. The golf ball 2 in which an average value ((L1+L2)/2) is equal to or greater than 0.80 but equal to or less than 0.95 is suitable for many amateur golf players.

When the golf ball 2 in which the average value ((L1+L2)/2) is equal to or greater than 0.80 is hit by an amateur golf player, a high trajectory is achieved. Although the golf ball 2 conforms to the rules of the USGA about flight distance, the golf player feels satisfied with the trajectory. The golf ball 2 is excellent in trajectory feeling. In this respect, the average value ((L1+L2)/2) is more preferably equal to or greater than 0.84 and particularly preferably equal to or greater than 0.88.

The golf ball 2 in which the average value ((L1+L2)/2) is equal to or less than 0.95 is less likely to rise during flight. In this respect, the average value ((L1+L2)/2) is more preferably equal to or less than 0.93 and particularly preferably equal to or less than 0.92.

The average value ((L1+L2)/2) can be achieved in the above range by making the specifications of the dimples 12 appropriate. Specifically, the ratio L1 and the ratio L2 can be made appropriate to achieve the average value ((L1+L2)/2) in the above range, by means such as:

(1) making the depth of each dimple 12 appropriate;

(2) making the area of each dimple 12 appropriate;

(3) making the volume of each dimple 12 appropriate;

(4) making the number of the dimples 12 appropriate;

(5) making the occupation ratio of the dimples 12 appropriate; and the like.

The ratio L1 is preferably equal to or greater than 0.85 but equal to or less than 0.93. When the golf ball 2 having a ratio L1 of equal to or greater than 0.85 is hit with a driver at a head speed of 35 m/s, a high trajectory is achieved. Although the golf ball 2 conforms to the rules of the USGA about flight distance, the golf player feels satisfied with the trajectory. The golf ball 2 is excellent in trajectory feeling. In this respect, the ratio L1 is more preferably equal to or greater than 0.87 and particularly preferably equal to or greater than 0.88. The golf ball 2 having a ratio L1 of equal to or less than 0.93 is less likely to rise during flight. In this respect, the ratio L1 is particularly preferably equal to or less than 0.92.

The ratio L2 is preferably equal to or greater than 0.76 but equal to or less than 0.92. When the golf ball 2 having a ratio L2 of equal to or greater than 0.76 is hit with a driver at a head speed of 45 m/s, a high trajectory is achieved. Although the golf ball 2 conforms to the rules of the USGA about flight distance, the golf player feels satisfied with the trajectory. The golf ball 2 is excellent in trajectory feeling. In this respect, the ratio L2 is more preferably equal to or greater than 0.80 and particularly preferably equal to or greater than 0.86. The golf ball 2 having a ratio L2 of equal to or less than 0.92 is less likely to rise during flight. In this respect, the ratio L2 is particularly preferably equal to or less than 0.91.

EXAMPLES Example 1

A rubber composition was obtained by kneading 100 parts by weight of a high-cis polybutadiene (trade name “BR-730”, manufactured by JSR Corporation), 22.5 parts by weight of zinc diacrylate, 5 parts by weight of zinc oxide, an appropriate amount of barium sulfate, 0.5 parts by weight of diphenyl disulfide, and 0.6 parts by weight of dicumyl peroxide. This rubber composition was placed into a mold including upper and lower mold halves each having a hemispherical cavity, and heated at 170° C. for 18 minutes to obtain a center with a diameter of 38.5 mm.

A resin composition was obtained by kneading 50 parts by weight of an ionomer resin (trade name “Himilan 1605”, manufactured by Du Pont-MITSUI POLYCHEMICALS Co., Ltd.), 50 parts by weight of another ionomer resin (“Himilan AM7329”, manufactured by Du Pont-MITSUI POLYCHEMICALS Co., Ltd.), and 4 parts by weight of titanium dioxide with a twin-screw kneading extruder. The center was covered with the resin composition by injection molding to form a mid layer with a thickness of 1.6 mm.

A paint composition (trade name “POLIN 750LE”, manufactured by SHINTO PAINT CO., LTD.) including a two-component curing type epoxy resin as a base polymer was prepared. The base material liquid of this paint composition includes 30 parts by weight of a bisphenol A type solid epoxy resin and 70 parts by weight of a solvent. The curing agent liquid of this paint composition includes 40 parts by weight of a modified polyamide amine, 55 parts by weight of a solvent, and 5 parts by weight of titanium dioxide. The weight ratio of the base material liquid to the curing agent liquid is 1/1. This paint composition was applied to the surface of the mid layer with a spray gun, and kept at 23° C. for 6 hours to obtain a reinforcing layer with a thickness of 10 μm.

A resin composition was obtained by kneading 100 parts by weight of a thermoplastic polyurethane elastomer (trade name “Elastollan XNY85A”, manufactured by BASF Japan Ltd.) and 4 parts by weight of titanium dioxide with a twin-screw kneading extruder. Half shells were formed from this resin composition by compression molding. The sphere consisting of the center, the mid layer, and the reinforcing layer was covered with two of these half shells. The half shells and the sphere were placed into a final mold that includes upper and lower mold halves each having a hemispherical cavity and having a large number of pimples on its cavity face, and a cover was obtained by compression molding. The thickness of the cover was 0.5 mm. Dimples having a shape that is the inverted shape of the pimples were formed on the cover. A clear paint including a two-component curing type polyurethane as a base material was applied to this cover to obtain a golf ball of Example 1 with a diameter of about 42.7 mm and a weight of about 45.6 g. The golf ball has dimple specifications D3 shown in Table 1 below.

Examples 2 to 11 and Comparative Examples 1 to 9

Golf balls of Examples 2 to 11 and Comparative Examples 1 to 9 were obtained in the same manner as Example 1, except the specifications of the center, the mid layer, the cover, and the dimples were as shown in Tables 5 to 8 below. The composition of the center is shown in detail in Table 1 below. The compositions of the mid layer and the cover are shown in detail in Table 2 below. The specifications of the dimples are shown in detail in Tables 3 and 4 below.

[Flight Test]

A driver with a head made of a titanium alloy (trade name “XXIO”, manufactured by DUNLOP SPORTS CO. LTD., shaft hardness: R, loft angle: 11°) was attached to a swing machine manufactured by Golf Laboratories, Inc. A golf ball was hit under a condition of a head speed of 40 m/sec, and a ball speed, a launch angle, a spin rate, highest point coordinates (x,y), and a carry were measured. The coordinate x is the horizontal distance from the launch point to the highest point. The coordinate y is the vertical distance from the launch point to the highest point. The carry is the distance from the launch point to the landing point. The results are shown in Tables 9 to 12 below.

[Sensuous Evaluation]

Ten testers hit golf balls with drivers and evaluated trajectory feeling and hit feeling. The evaluation was categorized on the basis of the following criteria. The results are shown in Tables 9 to 12 below.

Trajectory feeling: the number of testers who feel that a flight distance is long.

-   -   A: 8 or more     -   B: 5 to 7     -   C: 2 to 4     -   D: 1 or less

Hit feeling: the number of tester who feel soft.

-   -   A: 8 or more     -   B: 5 to 7     -   C: 2 to 4     -   D: 1 or less

TABLE 1 Composition of Center (parts by weight) T1 T2 T3 T4 T5 BR730 100 100 100 100 100 Zinc 18.5 20.0 22.5 26.5 27.5 diacrylate Zinc oxide 5 5 5 5 5 Barium * * * * * sulfate Diphenyl 0.5 0.5 0.5 0.5 0.5 disulfide Dicumyl 0.5 0.5 0.6 0.7 0.7 peroxide *: Appropriate amount

TABLE 2 Compositions of Mid Layer and Cover (parts by weight) M1 C1 C2 C3 C4 C5 Himilan 50 #1605 Himilan 50 50 AM7329 Himilan 24 AM7337 Rabalon 26 T3221C Barium 17 sulfate Elastollan 100 XNY82A Elastollan 100 XNY85A Elastollan 100 XNY90A Elastollan 100 XNY97A Titanium 4 4 4 4 4 6 dioxide Hardness 65 29 32 38 47 52 (Shore D)

TABLE 3 Specifications of Dimples Number of Diameter Depth Volume Type dimples (mm) (mm) (mm³) D1 A 42 4.6 0.224 1.865 B 72 4.4 0.214 1.626 C 66 4.2 0.183 1.272 D 126 4.0 0.164 1.030 E 12 3.9 0.149 0.892 F 12 2.6 0.100 0.265 D2 A 42 4.6 0.244 2.032 B 72 4.4 0.234 1.778 C 66 4.2 0.203 1.411 D 126 4.0 0.184 1.156 E 12 3.9 0.169 1.011 F 12 2.6 0.120 0.318 D3 A 42 4.6 0.264 2.198 B 72 4.4 0.254 1.931 C 66 4.2 0.223 1.550 D 126 4.0 0.204 1.282 E 12 3.9 0.189 1.131 F 12 2.6 0.140 0.371 D4 A 42 4.6 0.274 2.282 B 72 4.4 0.264 2.007 C 66 4.2 0.233 1.619 D 126 4.0 0.214 1.345 E 12 3.9 0.199 1.191 F 12 2.6 0.150 0.398

TABLE 4 Specifications of Dimples Number of Diameter Depth Volume Type dimples (mm) (mm) (mm³) D5 A 42 4.6 0.284 2.365 B 72 4.4 0.274 2.083 C 66 4.2 0.243 1.689 D 126 4.0 0.224 1.408 E 12 3.9 0.209 1.251 F 12 2.6 0.160 0.425 D6 A 42 4.6 0.294 2.449 B 72 4.4 0.284 2.160 C 66 4.2 0.253 1.759 D 126 4.0 0.234 1.471 E 12 3.9 0.219 1.311 F 12 2.6 0.170 0.451 D7 A 42 4.6 0.334 2.783 B 72 4.4 0.324 2.466 C 66 4.2 0.293 2.038 D 126 4.0 0.274 1.724 E 12 3.9 0.259 1.551 F 12 2.6 0.210 0.559

TABLE 5 Specifications of Golf Ball Comp. Ex. 3 Ex. 1 Ex. 2 Ex. 3 Ex. 4 Center Composition T3 T3 T3 T3 T3 Diameter 38.50 38.50 38.50 38.50 38.50 H1 (D) 46 46 46 46 46 DF1 (mm) 5.05 5.05 5.05 5.05 5.05 Weight (g) 35.0 35.0 35.0 35.0 35.0 Mid Composition M1 M1 M1 M1 M1 layer Thickness 1.60 1.60 1.60 1.60 1.60 (mm) H2 (D) 65 65 65 65 65 Diameter 41.70 41.70 41.70 41.70 41.70 Core DF2 (mm) 4.30 4.30 4.30 4.30 4.30 Weight (g) 43.4 43.4 43.4 43.4 43.4 Cover Composition C2 C2 C2 C2 C2 Thickness 0.50 0.50 0.50 0.50 0.50 (mm) H3 (D) 32 32 32 32 32 Ball Diameter 42.70 42.70 42.70 42.70 42.70 DF3 (mm) 4.10 4.10 4.10 4.10 4.10 Weight (g) 45.6 45.6 45.6 45.6 45.6 Dimple Specifications D1 D2 D3 D4 D5 Total volume 423 468 513 536 558 (mm³) CD1 0.311 0.287 0.274 0.238 0.233 CL1 0.301 0.266 0.252 0.211 0.198 L1 0.968 0.928 0.918 0.887 0.852 CD2 0.255 0.243 0.229 0.225 0.224 CL2 0.241 0.221 0.196 0.180 0.171 L2 0.945 0.912 0.860 0.801 0.761 (L1 + L2)/2 0.956 0.920 0.889 0.844 0.806

TABLE 6 Specifications of Golf Ball Comp. Comp. Comp. Ex. 4 Ex. 5 Ex. 6 Ex. 5 Ex. 6 Center Composition T3 T3 T1 T2 T4 Diameter 38.50 38.50 38.50 38.50 38.50 H1 (D) 46 46 38 42 51 DF1 (mm) 5.05 5.05 6.05 5.75 4.05 Weight (g) 35.0 35.0 35.0 35.0 35.0 Mid Composition M1 M1 M1 M1 M1 layer Thickness 1.60 1.60 1.60 1.60 1.60 (mm) H2 (D) 65 65 65 65 65 Diameter 41.70 41.70 41.70 41.70 41.70 Core DF2 (mm) 4.30 4.30 5.30 5.00 3.30 Weight (g) 43.4 43.4 43.4 43.4 43.4 Cover Composition C2 C2 C2 C2 C2 Thickness 0.50 0.50 0.50 0.50 0.50 (mm) H3 (D) 32 32 32 32 32 Ball Diameter 42.70 42.70 42.70 42.70 42.70 DF3 (mm) 4.10 4.10 5.10 4.80 3.10 Weight (g) 45.6 45.6 45.6 45.6 45.6 Dimple Specifications D6 D7 D3 D3 D3 Total volume 581 672 513 513 513 (mm³) CD1 0.235 0.238 0.274 0.274 0.274 CL1 0.197 0.169 0.252 0.252 0.252 L1 0.839 0.709 0.918 0.918 0.918 CD2 0.228 0.235 0.229 0.229 0.229 CL2 0.170 0.155 0.196 0.196 0.196 L2 0.745 0.661 0.860 0.860 0.860 (L1 + L2)/2 0.792 0.685 0.889 0.889 0.889

TABLE 7 Specifications of Golf Ball Comp. Comp. Comp. Ex. 7 Ex. 8 Ex. 7 Ex. 8 Ex. 9 Center Composition T5 T3 T3 T3 T3 Diameter 38.50 38.50 38.50 38.50 38.50 H1 (D) 54 46 46 46 46 DF1 (mm) 3.85 5.05 5.05 5.05 5.05 Weight (g) 35.0 35.0 35.0 35.0 35.0 Mid Composition M1 M1 M1 M1 M1 layer Thickness 1.60 1.60 1.60 1.60 1.60 (mm) H2 (D) 65 65 65 65 65 Diameter 41.70 41.70 41.70 41.70 41.70 Core DF2 (mm) 3.10 4.30 4.30 4.30 4.30 Weight (g) 43.4 43.4 43.4 43.4 43.4 Cover Composition C2 C1 C3 C4 C5 Thickness 0.50 0.50 0.50 0.50 0.50 (mm) H3 (D) 32 29 38 47 53 Ball Diameter 42.70 42.70 42.70 42.70 42.70 DF3 (mm) 2.85 4.15 4.05 4.00 3.95 Weight (g) 45.6 45.6 45.6 45.6 45.6 Dimple Specifications D3 D3 D3 D3 D3 Total volume 513 513 513 513 513 (mm³) CD1 0.274 0.274 0.274 0.274 0.274 CL1 0.252 0.252 0.252 0.252 0.252 L1 0.918 0.918 0.918 0.918 0.918 CD2 0.229 0.229 0.229 0.229 0.229 CL2 0.196 0.196 0.196 0.196 0.196 L2 0.860 0.860 0.860 0.860 0.860 (L1 + L2)/2 0.889 0.889 0.889 0.889 0.889

TABLE 8 Specifications of Golf Ball Comp. Comp. Ex. 9 Ex. 10 Ex. 11 Ex. 1 Ex. 2 Center Composition T3 T3 T3 T4 T4 Diameter 37.50 36.50 35.50 38.50 38.50 H1 (D) 46 46 46 51 51 DF1 (mm) 5.05 5.05 5.05 4.05 4.05 Weight (g) 32.3 29.8 27.4 35.0 35.0 Mid Composition M1 M1 M1 M1 M1 layer Thickness 1.60 1.60 1.60 1.60 1.60 (mm) H2 (D) 65 65 65 65 65 Diameter 40.70 39.70 38.70 41.70 41.70 Core DF2 (mm) 4.30 4.30 4.30 3.30 3.30 Weight (g) 40.3 37.4 34.6 43.4 43.4 Cover Composition C4 C4 C4 C3 C3 Thickness 1.00 1.50 2.00 0.50 0.50 (mm) H3 (D) 47 47 47 38 38 Ball Diameter 42.70 42.70 42.70 42.70 42.70 DF3 (mm) 4.05 4.10 4.15 3.05 3.05 Weight (g) 45.6 45.6 45.6 45.6 45.6 Dimple Specifications D3 D3 D3 D6 D7 Total volume 513 513 513 581 672 (mm³) CD1 0.274 0.274 0.274 0.235 0.238 CL1 0.252 0.252 0.252 0.197 0.169 L1 0.918 0.918 0.918 0.839 0.709 CD2 0.229 0.229 0.229 0.228 0.235 CL2 0.196 0.196 0.196 0.170 0.155 L2 0.860 0.860 0.860 0.745 0.661 (L1 + L2)/2 0.889 0.889 0.889 0.792 0.685

TABLE 9 Results of Evaluation Comp. Ex. Ex. Ex. Ex. Ex. 3 1 2 3 4 Cover thickness 0.50 0.50 0.50 0.50 0.50 (mm) Hardness H3 32 32 32 32 32 (D) Deformation 4.10 4.10 4.10 4.10 4.10 DF3 (mm) Total volume 423 468 513 536 558 (mm³) L1 0.968 0.928 0.918 0.887 0.852 L2 0.945 0.912 0.860 0.801 0.761 (L1 + L2)/2 16.473 16.456 16.430 16.400 16.381 Flight test Ball speed 56.80 56.80 56.80 56.80 56.80 (m/s) Launch angle 12.5 12.5 12.5 12.5 12.5 (deg) Spin (rpm) 2850 2850 2850 2850 2850 Highest point 102 107 111 109 111 (x) Highest point 24 23 22 21 19 (y) Carry (m) 162 166 168 176 177 Feeling Trajectory C B A A B feeling Hit feeling A A A A A

TABLE 10 Results of Evaluation Comp. Comp. Comp. Ex. Ex. Ex. Ex. Ex. 4 5 6 5 6 Cover thickness 0.50 0.50 0.50 0.50 0.50 (mm) Hardness H3 32 32 32 32 32 (D) Deformation 4.10 4.10 5.10 4.80 3.10 DF3 (mm) Total volume 581 672 513 513 513 (mm³) L1 0.839 0.709 0.918 0.918 0.918 L2 0.745 0.661 0.860 0.860 0.860 (L1 + L2)/2 16.372 16.330 16.430 16.430 16.430 Flight test Ball speed 56.80 56.80 55.70 56.30 57.45 (m/s) Launch angle 12.5 12.5 12.8 12.7 12.4 (deg) Spin (rpm) 2850 2850 2700 2750 2900 Highest point 113 107 107 107 111 (x) Highest point 18 17 21 22 23 (y) Carry (m) 179 173 164 166 170 Feeling Trajectory C D A A A feeling Hit feeling A A C B B

TABLE 11 Results of Evaluation Comp. Comp. Comp. Ex. Ex. Ex. Ex. Ex. 7 8 7 8 9 Cover thickness 0.50 0.50 0.50 0.50 0.50 (mm) Hardness H3 32 29 38 47 53 (D) Deformation 2.85 4.15 4.05 4.00 3.95 DF3 (mm) Total volume 513 513 513 513 513 (mm³) L1 0.918 0.918 0.918 0.918 0.918 L2 0.860 0.860 0.860 0.860 0.860 (L1 + L2)/2 16.430 14.930 19.430 23.930 26.930 Flight test Ball speed 57.55 56.75 56.85 56.90 56.95 (m/s) Launch angle 12.3 12.3 13.1 13.4 13.5 (deg) Spin (rpm) 2950 2950 2550 2400 2350 Highest point 111 108 109 112 114 (x) Highest point 23 22 22 22 22 (y) Carry (m) 170 165 171 175 178 Feeling Trajectory A C A A A feeling Hit feeling D A A A A

TABLE 12 Results of Evaluation Comp. Comp. Ex. Ex. Ex. Ex. Ex. 9 10 11 1 2 Cover thickness 1.00 1.50 2.00 0.50 0.50 (mm) Hardness H3 47 47 47 38 38 (D) Deformation 4.05 4.10 4.15 3.05 3.05 DF3 (mm) Total volume 513 513 513 581 672 (mm³) L1 0.918 0.918 0.918 0.839 0.709 L2 0.860 0.860 0.860 0.745 0.661 (L1 + L2)/2 23.930 23.930 23.930 19.372 19.330 Flight test Ball speed 56.85 56.80 56.75 57.50 57.50 (m/s) Launch angle 13.0 12.8 12.7 13.0 13.0 (deg) Spin (rpm) 2600 2700 2750 2600 2600 Highest point 111 110 108 115 108 (x) Highest point 22 22 22 19 18 (y) Carry (m) 169 168 166 183 176 Feeling Trajectory A A B A D feeling Hit feeling A A B C C

As shown in Tables 9 to 12, the golf ball of each Example is excellent in various performance characteristics. From the results of evaluation, advantages of the present invention are clear.

The golf ball according to the present invention is suitable for, for example, playing golf on golf courses and practicing at driving ranges. The above descriptions are merely illustrative examples, and various modifications can be made without departing from the principles of the present invention. 

What is claimed is:
 1. A golf ball comprising a core and a cover positioned outside the core, wherein the golf ball has a large number of dimples on a surface thereof, the cover has a Shore D hardness of equal to or greater than 30 but equal to or less than 50, the golf ball has an amount of compressive deformation of equal to or greater than 3.0 mm but equal to or less than 5.0 mm, the amount of compressive deformation being measured under conditions of an initial load of 98 N and a final load of 1274 N, and the golf ball meets the following mathematical formula (I): 0.80≦((L1+L2)/2)≦0.95  (I), where: L1 represents a ratio (CL1/CD1) of a lift coefficient CL1 relative to a drag coefficient CD1, the lift coefficient CL1 and the drag coefficient CD1 being measured under conditions of a Reynolds number of 1.290×10⁵ and a spin rate of 2820 rpm; and L2 represents a ratio (CL2/CD2) of a lift coefficient CL2 relative to a drag coefficient CD2, the lift coefficient CL2 and the drag coefficient CD2 being measured under conditions of a Reynolds number of 1.771×10⁵ and a spin rate of 2940 rpm.
 2. The golf ball according to claim 1, wherein a total volume of the dimples is equal to or greater than 430 mm³ but equal to or less than 580 mm³.
 3. The golf ball according to claim 1, wherein the cover has a thickness of equal to or greater than 0.3 mm but equal to or less than 1.8 mm.
 4. The golf ball according to claim 1, wherein the ratio L1 is equal to or greater than 0.85 but equal to or less than 0.93.
 5. The golf ball according to claim 1, wherein the ratio L2 is equal to or greater than 0.76 but equal to or less than 0.92.
 6. The golf ball according to claim 1, wherein a ratio of a sum of spherical surface areas of the dimples relative to a surface area of a phantom sphere of the golf ball is equal to or greater than 0.780 but equal to or less than 0.950.
 7. The golf ball according to claim 1, wherein a total number of the dimples is equal to or greater than 250 but equal to or less than
 450. 8. The golf ball according to claim 1, wherein each dimple has a diameter Dm of equal to or greater than 2.0 mm but equal to or less than 6.0 mm.
 9. The golf ball according to claim 1, wherein each dimple has a depth of equal to or greater than 0.10 mm but equal to or less than 0.60 mm. 